Dc-dc converter

ABSTRACT

A DC-DC converter includes an error amplifying circuit to output an error signal by amplifying differences between a reference voltage and a feedback voltage for an output voltage generated by switching an output transistor between on and off, a triangular wave generation circuit to generate a triangular wave, a PWM comparison circuit to compares the triangular wave with the error signal and output a duty control signal having a duty ratio based on the comparison, a pulse width control circuit to control a pulse width of the duty control signal output from the PWM comparison circuit, and a drive circuit to drive the output transistor according to a signal output from the pulse width control circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent specification claims priority from Japanese PatentApplication No. 2008-060005, filed on Mar. 10, 2008 in the Japan PatentOffice, the entire contents of which are hereby incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a DC-DC (direct current todirect-current) converter that controls operations of a switchingelement based on an output voltage and maintains the output voltagewithin a predetermined range.

More particularly, the present invention relates to a DC-DC converterthat supplies operational power to various types of semiconductorintegrated circuit (IC) devices, such as a central processing unit (CPU)and a memory (RAM, ROM, etc.), mounted on any of various types ofelectronic apparatuses. For example, applications thereof can be a powersource for digital home appliance such as DVD players using both blue-and red-type LD, and DVD-ROM/R/RW using both blue- and red-type LD fornotebook PCs and desktop PCs.

2. Discussion of the Background

These days, as typified by cell phones, small mobile devices areconsiderably popular, and secondary batteries are used as a power supplyfor such small mobile device. Further, the above-described electronicapparatuses include a great number of ICs, and these ICs require powerrespective supply. The power supply required for operations of a load isgenerated in a DC-DC converter. If the power supply becomes unstable,operations of the load become unstable, and accordingly, malfunctionwill occur. Therefore, the DC-DC converter needs to generate a stablevoltage as the power supply consistently.

FIG. 8 shows circuitry of a known DC-DC converter 201. The DC-DCconverter 201 includes a control circuit 202 formed on a one-chip ICdevice, and multiple external elements. A signal SG1 output from thecontrol circuit 202 is supplied to a gate of an output transistor 203that is an enhanced N-channel MOS (negative channel metal oxidesemiconductor) transistor.

A supply voltage input terminal VIN is connected to one terminal of acoil 204 for increasing voltage, and the other terminal of the coil 204is connected to a drain of the output transistor 203. A source of theoutput transistor 203 is connected to ground (GND). Additionally, adiode 205 is connected between the drain and the source of the outputtransistor 203. More specifically, an anode of the diode 205 isconnected to the source of the transistor 203, and a cathode of thediode 205 is connected to the drain of the transistor 203.

A junction node between the coil 204 and the output transistor 203 isconnected to an anode of a diode 206, and a cathode of the diode 206 isconnected to an output terminal 208. The output terminal 208 isconnected to ground GND via a smoothing capacitor 207. In other words, asmoothing circuit formed by both the smoothing capacitor 207 and thecoil 204 smoothes an output voltage VOUT. Further, the output terminal208 is connected to the control circuit 202, and the output voltage VOUTis provided to the control circuit 202.

The control circuit 202 includes an error amplification circuit 211, areference-voltage generation circuit 215, a PWM (Pulse Width Modulation)comparison circuit 212, a triangular-wave generation circuit 213, and anoutput control circuit 214.

The error amplification circuit 211 has an inverting input terminal, anon-inverting input terminal, and an output terminal. The invertinginput terminal of the error amplification circuit 211 receives a voltageVDIV generated by dividing the output voltage VOUT from the outputterminal 208 by feedback resistors 216 and 217. The non-inverting inputterminal of the error amplification circuit 211 receives a referencevoltage VREF from the reference-voltage generation circuit 215.

In the error amplification circuit 211, a series circuit, not shown,that includes a phase compensating capacitor and a resistor is connectedbetween the output terminal of the error amplification circuit 211 andthe inverting input terminal, thereby preventing oscillation of theerror amplification circuit 211.

The error amplification circuit 211 compares the reference voltage VREFwith the voltage VDIV that is generated by dividing output voltage VOUTby feedback resistors 216 and 217 with the reference voltage VREF.Therefore, the error amplification circuit 211 generates an error outputsignal SG2 by amplifying differences between the voltage VDIV and VREF,and outputs the error output signal SG2 to the PWM comparison circuit212, in a subsequent stage.

The PWM comparison circuit 212 has an inverting input terminal, anon-inverting input terminal, and an output terminal. The non-invertinginput terminal of the PWM comparison circuit 212 receives the erroroutput signal SG2 from the error amplification circuit 211, and theinverting input terminal of the PWM comparison circuit 212 receives atriangular wave signal SG3 from the triangular-wave generation circuit213. The PWM comparison circuit 212 compares the error output signal SG2and the triangular wave signal SG3.

The PWM comparison circuit 212 outputs a pulse signal to the outputcontrol circuit 214 as a duty control signal SG4 that turns low when thetriangular wave signal SG3 has a voltage higher than the error outputsignal SG2, and turns high when the triangular wave signal SG3 has avoltage equal to or smaller than the error output signal SG2.

The output control circuit 214 outputs to the gate of the outputtransistor 203 the duty control signal SG4 from the PWM comparisoncircuit 212, as above-described output signal SG1.

The DC-DC converter 201 having the configuration described aboveswitches the output transistor 203 between on and off based on theoutput signal SG1 output from the control circuit 202, thereby keepingthe output voltage VOUT at a predetermined voltage value.

When the output voltage decreases, the level of the error output signalSG2 of the error amplification circuit 211 rises. On the other hand,when the output voltage increases, the level of the error output signalSG2 of the error amplification circuit 211 declines.

In the PWM comparison circuit 212, when the level of the error outputsignal SG2 rises, the period during which the level of the triangularwave signal SG3 is higher than that of the error output signal SG2 isshorter, therefore, a period during which the duty control signal SG4 ishigh (hereinafter “high level signal period”) becomes longer. That is, aduty ratio of signal SG4 increases.

By contrast, in the PWM comparison circuit 212, when the level of theerror output signal SG2 declines, the period during which the signallevel of the triangular wave signal SG3 is higher than that of the erroroutput signal SG2 is longer, therefore, the high level signal period ofthe duty control signal SG4 becomes shorter. That is, the duty ratio ofsignal SG4 decreases.

Further, as shown in FIG. 9B, when a slope of the triangular wave outputfrom the triangular-wave generation circuit 213 shows an identicallinearity, it is possible to obtain the stable output voltage byfeedback of the output voltage VOUT. However, actually, as shown inFIGS. 9A and 9B, the slope of the triangular wave is a non-linear. Dueto the non-linearity of the slope, the pulse width of a subsequent dutycontrol signal from the PWM comparison circuit 212 fluctuates, which isa problem.

For example, as shown in FIG. 9A, when the output voltage VOUT is high,the DC-DC converter needs to reduce the pulse width, and therefore, thelevel of the error output signal SG2 is lowered. After one clock (CLK),the level of the error output signal SG2 is lowered, and at a non-linearpart of the slope of the triangular wave, the pulse width becomesshorter than that for the identical linearity shown in FIG. 9B.

By contrast, when the triangular wave has the slope shapes shown in FIG.9C, the pulse width expands to become greater than that for theidentical linearity shown in FIG. 9B.

As described above, since the output triangle slope has such anon-linear shape, it can take a longer time to converge on a requiredvoltage, and at worst, the DC-DC converter can fail to generate therequired output voltage.

Several approaches have been tried in an attempt to make the DC-DCconverter generate a stable voltage consistently.

In one known configuration of the DC-DC converter, when an outputvoltage is lower than a predetermined voltage that is offset from areference voltage, an output transistor is turned on while a dutycontrol signal has a maximum duty ratio by comparing an error outputsignal and a triangular wave signal. In this configuration, even if theoutput voltage drops below the predetermined voltage, the outputtransistor is kept on in the period during which the output voltage islow, and accordingly the output voltage can rise quickly.

However, although the above-mentioned approach considers the case inwhich the output voltage drops significantly, it does not consider thecase in which the triangular wave is not linear. Thus, a required pulsewidth signal cannot be generated in the case in which an error outputsignal is lower when the triangular wave is not linear.

SUMMARY OF THE INVENTION

In view of the forgoing, one illustrative embodiment of the presentinvention provides a DC-DC converter that includes an error amplifyingcircuit that outputs an error signal by amplifying a difference betweena reference voltage and a feedback voltage for an output voltagegenerated by switching an output transistor between on and off, atriangular wave generation circuit that generates a triangular wave, aPWM comparison circuit that compares the triangular wave and the errorsignal and outputs a duty control signal having a duty ratio based onthe comparison, a pulse width control circuit that controls a pulsewidth of the duty control signal output from the PWM comparison circuit,and a drive circuit that drives the output transistor according to asignal output from the pulse width control circuit.

Therefore, the present invention can provide a DC-DC converter that canperform reliable pulse-width modulation to output a stable outputvoltage by controlling the modulation of the pulse widths using thelinear part of the triangular wave.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 illustrates circuitry of a DC-DC converter according to anillustrative embodiment of the present invention;

FIG. 2 is a diagram illustrating circuitry of a pulse-width controlcircuit shown in FIG. 1;

FIG. 3 is a waveform diagram illustrating operations of the DC-DCconverter shown in FIG. 1;

FIG. 4 is a diagram illustrating circuitry of a pulse-width controlcircuit according to another illustrative embodiment;

FIG. 5 is a waveform diagram illustrating operations of a DC-DCconverter using the pulse-width control circuit shown in FIG. 4;

FIG. 6 is circuitry of a DC-DC converter according to anotherillustrative embodiment;

FIG. 7 is a diagram illustrating circuitry of a pulse-width controlcircuit shown in FIG. 6;

FIG. 8 is circuitry illustrating a known DC-DC converter; and

FIG. 9A, 9B, and 9C are waveform diagrams illustrating operations of theDC-DC converter shown in FIG. 8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In describing preferred embodiments illustrated in the drawings,specific terminology is employed for the sake of clarity. However, thedisclosure of this patent specification is not intended to be limited tothe specific terminology so selected and it is to be understood thateach specific element includes all technical equivalents that operate ina similar manner and achieve a similar result.

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views thereof,particularly to FIGS. 1 through 3, a DC-DC converter according to anexample embodiment of the present invention is described below.

FIG. 1 illustrates circuitry of a DC-DC converter 1 according to thepresent embodiment. The DC-DC converter 1 includes a control circuit 2formed on a one-chip IC device, and multiple external elements. A signalSG1 output from the control circuit 2 is supplied to a gate of an outputtransistor 3 that is an enhanced N-channel MOS transistor.

A supply voltage input terminal VIN is connected to one terminal of acoil 4 for increasing voltage, and the other terminal of the coil 4 isconnected to a drain of the output transistor 3. A source of the outputtransistor 3 is connected to ground (GND).

Additionally, a diode 5 is connected between the drain and the source ofthe output transistor 3, more specifically, an anode of the diode 5 isconnected to the source of the transistor 3, and a cathode of the diode5 is connected to the drain of the transistor 3.

A junction node between the coil 4 and the output transistor 3 isconnected to an anode of a diode 6, and a cathode of the diode 6 isconnected to an output terminal 8.

The output terminal 8 is connected to the ground GND via a smoothingcapacitor 7. In other words, a smoothing circuit formed by both thesmoothing capacitor seven and the coil 4 smoothes an output voltageVOUT.

Further, the output terminal 8 is connected to the control circuit 2,and the output voltage VOUT is provided to the control circuit 2.

The control circuit 2 includes an error amplification circuit 11, areference voltage generation circuit 15, a PWM (Pulse Width Modulation)comparison circuit 12, a triangular-wave generation circuit (oscillator)13, a pulse-width control circuit 18A, and an output control circuit 14.

The error amplification circuit 11 has an inverting input terminal, anon-inverting input terminal, and an output terminal. The invertinginput terminal of the error amplification circuit 11 receives a voltageVdiv generated by dividing the output voltage VOUT (serves as a feedbackvoltage VFB) output from the output terminal 8 by feedback resistors 16and 17.

The non-inverting input terminal of the error amplification circuit 11receives a reference voltage from the reference voltage generationcircuit 15. In the error amplification circuit 11, a series circuit, notshown, that includes a phase compensating capacitor and a resistor isconnected between the output terminal and the inverting input terminal,thereby preventing oscillation of the error amplification circuit 11.

The error amplification circuit 11 compares the reference voltage VREFwith the voltage Vdiv that is generated by dividing the output voltageVOUT by the feedback resistors 16 and 17. Therefore, the erroramplification circuit 11 produces an error output signal SG2 byamplifying differences between the voltage Vdiv and the referencevoltage VREF, and outputs the error output signal SG2 to the PWMcomparison circuit 12 in a subsequent stage.

The PWM comparison circuit 12 has an inverting input terminal, anon-inverting input terminal, and an output terminal. The non-invertinginput terminal of the PWM comparison circuit 12 receives the erroroutput signal SG2 from the error amplification circuit 11, and theinverting input terminal of the PWM comparison circuit 12 receives atriangular wave signal SG3 from the triangular-wave generation circuit13. The PWM comparison circuit 12 compares the error output signal SG2with the triangular wave signal SG3.

The PWM comparison circuit 12 outputs a pulse signal to the pulse-widthcontrol circuit 18A as a duty control signal SG4 that tunes low when thetriangular wave signal SG3 has a voltage higher than the error outputsignal SG2 and turns high when the triangular wave signal SG3 has avoltage equal to or smaller than the error output signal SG2.

The pulse-width control circuit 18A controls a pulse width of the dutycontrol signal SG4. In the present embodiment, the pulse-width controlcircuit 18A reduces the duty ratio of the duty control signal SG4 by 10%and outputs a control signal SG5 whose pulse width is controlled.

The output control circuit 14 serving as a drive circuit outputs to thegate of the output transistor 3 the control signal SG5 from thepulse-width control circuit 18A as the above-described signal SG1.

The DC-DC converter 1 having the configuration described above switchesthe output transistor 3 between on and off based on the signal SG1output from the control circuit 2. Therefore, the DC-DC converter 1keeps the output voltage VOUT at the predetermined voltage value.

When the output voltage VOUT decreases, the level of the error outputsignal SG2 of the error amplification circuit 11 rises.

On the other hand, when the output voltage increases, the level of theerror output signal SG2 of the error amplification circuit 11 declines.

In the PWM comparison circuit 12, when the level of the error outputsignal SG2 rises, the period during which the level of the triangularwave signal SG3 is higher than that of the error output signal SG2 isshorter. Therefore, a period during which the duty control signal SG4 ishigh (hereinafter “high level signal period”) becomes longer. That is, aduty ratio of SG4 increases.

By contrast, in the PWM comparison circuit 12, when the level of theerror output signal SG2 declines, the period during which the signallevel of the triangular wave signal SG3 is higher than that of the erroroutput signal SG2 is shorter. Therefore, the high level signal period ofthe duty control signal SG4 becomes shorter. That is, the duty ratio ofSG4 decreases.

In the present embodiment, the DC-DC converter 1 is set to performnegative feedback so that a duty ratio of a signal LX at a junction nodebetween the coil 4 and the output transistor 3 is kept at 15%. Thetriangular-wave generation circuit 13 outputs the triangular wave signalSG3 while operating at a frequency of 1 MHz, for example.

The pulse-width control circuit 18A is designed to control the dutycontrol signal SG4 so that its duty ratio is reduced by 10%. Thepulse-width control circuit 18A outputs the signal SG5 whose pulse widthis reduced by 10% from that of the duty control signal SG4, and thesignal SG5 controls the output control circuit 14. As a result, thelevel of the error output signal SG2 becomes higher by 10% than that ofthe signal LX.

Therefore, in the present embodiment, when the duty ratio of the signalLX is set to 15%, the error output signal SG2 is output at a duty ratioof 25%. Thus, the duty control signal SG4 of the PWM comparison circuit12 is output at a duty ratio of 25% as well.

Then, because the pulse-width control circuit 18A reduces the pulsewidth of the duty control signal SG4 by 10%, the signal SG5 output fromthe pulse-width control circuit 18A is output at a duty ratio of 15%.

FIG.2 shows an example of a configuration of the above-describedpulse-width control circuit 18A. FIG. 2 is a diagram illustratingcircuitry of the pulse-width control circuit 18A according to thepresent embodiment. The pulse-width control circuit 18A includes a delaycircuit 181 that delays the duty control signal SG4 for a predeterminedor given time, and an AND circuit 182 that receives the duty controlsignal SG4 and a signal SG81 from the delay circuit 181.

The AND circuit 182 outputs the signal SG5 whose pulse width iscontrolled so that its duty ratio is reduced by 10% from that of thesignal SG4. When the triangular-wave generation circuit 13 outputs thetriangular wave while operating at a frequency of 1 MHz, for example,the delay circuit 181 delays the signal SG81 for 100 ns in order toreduce the duty ratio of the signal SG5 by 10%.

As described above with reference to FIGS. 9A and 9C, the triangularwave slope in the triangular-wave generation circuit 13 has a non-linearpart.

In the cases shown in FIGS. 9A and 9C, when there are no differencesbetween the two signals that the error amplification circuit receives,in other words, between the input voltage and the output voltage, theerror output signal SG2 is provided to the non-linear part that is alower part of the slope of the triangular wave. In this case, asdescribed-above, the pulse width of the signal SG4 will be out of arequired pulse width.

Therefore, in the present embodiment, the pulse-width control circuit18A outputs the signal SG5 whose duty ratio is reduced by 10%. As aresult, the error output signal SG2 is raised, and accordingly, theerror output signal SG2 is provided to the linear part of the triangularwave that is output from the triangular-wave generation circuit 13.

FIG. 3 is a waveform diagram illustrating operations of the DC-DCconverter 1 of the present embodiment.

Referring to FIGS. 1 though 3, the operations of the DC-DC converter 1according to the present embodiment are described below. In FIG. 3, theerror output signal SG2 of the known DC-DC converter 201 shown in FIG. 8and the PWM output signal SG4 based on that error output signal SG2 arealso shown as a reference.

Referring to FIG. 3, a slope of the output signal SG3 (OSC outputsignal) of the triangular-wave generation circuit 13 is not an idealtriangular wave. More specifically, the non-linear part is situated at astart-up part and a nearing an end part in cycles of the triangularwave. If there are no differences between the input voltage and theoutput voltage, the error output signal SG2 becomes lower.

In the known DC-DC converter 201 shown in FIG. 8, the error outputsignal is compared with the non-liner part of the output of thetriangular-wave generation circuit 13, and as a result, the pulse widthof the PWM output shown as REFERENCE in FIG. 3 is narrower than therequired pulse width.

The required pulse to the identical slope is shown as a dashed line.

By contrast, in the DC-DC converter 1 according to the presentembodiment, the pulse-width control circuit 18A outputs the controlsignal SG5 whose pulse width is reduced by 10%, and this signal SG5controls the control circuit 14. As a result, the level of the erroroutput signal SG2 becomes higher by 10% than that of the signal LX.Therefore, in the present embodiment, when the duty ratio of the signalLX is set to 15%, the period during which the signal level of thetriangular wave signal SG3 is higher than that of error output signalSG2 is 25% of 1 cycle. Thus, the level of the error output signal SG2 inthe DC-DC converter 1 that includes the pulse-width control circuit 18reducing the duty ratio 10% is raised by 10% compared to the erroroutput signal SG2 in a DC-DC converter that does not have such apulse-width control circuit. AS a result, the duty control signal SG4,that is, the PWM output, of the PWM comparison circuit 12 is output at aduty ratio of 25% as well.

Subsequently, the signal SG4 is delayed by 10% in the delay circuit 181shown in FIG. 2 of the pulse-width control circuit 18A. Then, the dutycontrol signal SG4 and the output signal SG81 from the delay circuit 181are provided to the AND circuit 182, and thus, the AND circuit 182outputs the control signal SG5 whose duty ratio is 15%.

The control signal SG5 can be output at a pulse width identical orsimilar to the pulse width corresponding to the ideal slope of thetriangular wave output from the triangular-wave generation circuit 13.

Therefore, even if there are no differences between the input voltageand the output voltage, the PWM operation can be stable.

Another embodiment of a pulse-width control circuit 18B is describedbelow with reference to FIGS. 4 and 5.

Although its circuitry is similar to the circuitry of the DC-DCconverter 1 shown in FIG. 1, a DC-DC converter according to the presentembodiment has a pulse-width control circuit 18B that operatesdifferently from that shown in FIG. 2.

In the first embodiment, the pulse-width control circuit 18A reduces theduty ratio of the duty control signal SG4 by 10%.

By contrast, in the second embodiment, the pulse-width control circuit18B is designed to control the duty control signal SG4 so that it raisesthe duty ratio by 10% and outputs the signal SG5 whose pulse width iscontrolled. The pulse-width control circuit 18B outputs the signal SG5whose pulse width is increased by 10% from that of the duty controlsignal SG4, and the signal SG5 controls the output control circuit 14.As a result, the level of the error output signal SG2 becomes lower by10% than that of the signal LX.

Therefore, in the present embodiment, when the duty ratio of the signalLX is set to 95%, the period during which the signal level of thetriangular wave signal SG3 is higher than that of the error outputsignal SG2 is 85% of 1 cycle. Thus, the level of the error output signalSG2 in the DC-DC converter 1 that includes the pulse-width controlcircuit 18 raising the duty ratio 10% is reduced by 10% compared to theerror output signal SG2 in the DC-DC converter that does not have thepulse-width control circuit. Thus, the duty control signal SG4 of thePWM comparison circuit 12 can have a duty ratio of 85% as well. Then,because the pulse-width control circuit 18B raises the pulse width ofthe duty control signal SG4 by 10%, the signal SG5 output from thepulse-width control circuit 18B is output at a duty ratio of 95%.

In order to increase the duty ratio of the output signal SG5 to beprovided to the output control circuit 14, the signal SG5 is controlledby using an upper part of the triangular wave SG3.

As shown in FIG. 5, the upper part of the triangular wave also has anon-linear part, and accordingly, if this part is used for controllingthe pulse width of the signal SG4, the pulse width of the signal SG4will fluctuate from the pulse width corresponding to an ideal slope.

Therefore, in the second embodiment, when the pulse-width controlcircuit 18B provides the output control circuit 14 with the outputsignal SG5 whose duty ratio is higher, the pulse-width control circuit18B is designed to raise the duty ratio of the duty control signal SG4by 10% and to lower the level of the output signal SG2 so as to controlthe signal SG4 using the linear part of the triangular wave.

The pulse-width control circuit 18B operated as described above can beconfigured as shown in FIG. 4, for example. FIG. 4 is a diagramillustrating circuitry of the pulse-width control circuit 18B accordingto the second embodiment. The pulse-width control circuit 18B includes adelay circuit 181 that delays the duty control signal SG4 for apredetermined or given time, and an OR circuit 183 that receives theduty control signal SG4 and a signal from the delay circuit 181. The ORcircuit 183 outputs the signal SG5 whose pulse width is controlled sothat its duty ratio is increased by 10%.

When the triangular-wave generation circuit 13 outputs the triangularwave while operating at a frequency of 1 MHz, for example, the delaycircuit 181 delays the signal SG81 for 100 ns in order to raise the dutyratio of the signal SG5 by 10%.

FIG. 5 is a waveform diagram illustrating the operations of the DC-DCconverter 1 when the pulse-width control circuit 18B of the secondembodiment is used instead of the pulse-width control circuit 18A shownin FIG. 2.

Referring to FIGS. 1, 4 and 5, the operation of DC-DC converter 1according to the second embodiment is described below. In FIG. 5, theerror output signal SG2 of the known DC-DC converter 201 shown in FIG. 8and the PWM output signal SG4 based on the error output signal as areference.

Referring to FIG. 5, a slope of the output signal (OSC output signalSG3) of the triangular-wave generation circuit 13 is not the idealtriangular wave. More specifically, the non-linear part is situated at astart-up part and a nearing an end part in cycles of the triangularwave.

In the known DC-DC converter 201 shown in FIG. 8, the error outputsignal shown as reference SG2 is compared with the non-liner part of theoutput of the triangular-wave generation circuit 13.

By contrast, in the DC-DC converter 1 according to the secondembodiment, the pulse-width control circuit 18B outputs the controlsignal SG5 whose pulse width is increased by 10%, and this signal SG5controls the control circuit 14. As a result, the level of the erroroutput signal SG2 becomes lower by 10% than that of the signal LX.

Therefore, in the second embodiment, when the duty ratio of the signalLX is set to 95%, the error output signal SG2 has a duty ratio of 85%,and the error output signal is reduced. AS a result, the duty controlsignal SG4, that is PWM output, of the PWM comparison circuit 12 isoutput at the duty ratio of 85% as well.

Subsequently, the signal SG4 is delayed by 10% in the delay circuit 181shown in FIG. 4 of the pulse-width control circuit 18B. Then, the dutycontrol signal SG4 and the output signal SG81 from the delay circuit 181are provided to the OR circuit 183, and thus, the OR circuit 183 outputsthe control signal SG5 whose duty ratio is 95%.

The control signal SG5 can be output a pulse width identical or similarto the pulse width corresponding to the ideal slope of the pulse widthfrom triangular-wave generation circuit 13.

FIG. 6 illustrates circuitry of a DC-DC converter 1A according to thethird embodiment of the present invention. FIG. 7 is a diagramillustrating circuitry of a pulse-width control circuit 18C used in theDC-DC converter 1A shown in FIG. 6 according to the third embodiment.The DC-DC converter 1A includes a control circuit 20 and detectionscircuits 21 and 22 in addition to the elements included in the DC-DCconverter 1 shown in FIG. 1.

In the third embodiment, the pulse-width control circuit 18C includes afirst circuit, a second circuit, and a selector 185. The first circuitconsists of a delay circuit 181 and an AND circuit 182, and outputs thesignal SG5 whose pulse width is reduced by a predetermined or givenamount. The second circuit consists of the delay circuit 181 and an ORcircuit 183, and outputs the signal SG5 whose pulse width is raised by apredetermined or given amount.

The selector 185 selects either the first circuit or the second circuitand outputs the signal SG5 that is output from the selected circuit. Theselector 185 is controlled based on the input voltage and the outputvoltage.

Therefore, in the present embodiment, the input voltage and the outputvoltage is detected by the detection circuits 21 and 22, and signalsfrom these detection circuits 21 and 22 are provided to the controller20.

Based on the relation between the input voltage and the output voltage,the controller 20 controls the selector 185 to select the first circuitwhen the error output signal is situated in a portion lower than thecenter part of the triangular wave and to select the second circuit whenthe error output signal is situated in a portion above the center partof the triangular wave.

By controlling the pulse widths as described above, modulation of pulsewidth using the linear part of the triangular wave can be performed.Therefore, the present invention can provide a DC-DC converter that canperform reliable pulse-width modulation to output a stable outputvoltage.

It is to be noted that although the description above concerns examplesin which the pulse-width control circuit increases or decreases the dutyratio by 10%, such is but example thereof and the present invention isnot limited to the above-descried embodiments. That is, the amount bywhich the duty ratio can be adjusted depends on the circuits, theapplications for which the DC-DC converter is used, and the like.

Numerous additional modifications and variations are possible in lightof the above teachings. It is therefore to be understood that, withinthe scope of the appended claims, the disclosure of this patentspecification may be practiced otherwise than as specifically describedherein. Further, the claims are also intended to various alternations indoctrine of equivalents to the appended claim.

1. A DC-DC converter comprising: an error amplifying circuit to outputan error signal by amplifying a difference between a reference voltageand a feedback voltage for an output voltage generated by switching anoutput transistor between on and off; a triangular wave generationcircuit to generate a triangular wave; a PWM comparison circuit tocompare the triangular wave with the error signal and output a dutycontrol signal having a duty ratio based on the comparison; a pulsewidth control circuit to control a pulse width of the duty controlsignal output from the PWM comparison circuit; and a drive circuit todrive the output transistor according to a signal output from the pulsewidth control circuit.
 2. The DC-DC converter according to claim 1,wherein the pulse width control circuit reduces the pulse width of theduty control signal output from the PWM comparison circuit by apredetermined amount.
 3. The DC-DC converter according to claim 2,wherein the pulse width control circuit further comprises: a delaycircuit to delay the duty control signal for a predetermined time; andan AND circuit to receive the duty control signal and a signal from thedelay circuit.
 4. The DC-DC converter according to claim 1, wherein thepulse width control circuit increases the pulse width of duty controlsignal output from the PWM comparison circuit by a predetermined amount.5. The DC-DC converter according to claim 4, wherein the pulse widthcontrol circuit further comprise: a delay circuit to delay the dutycontrol signal for a predetermined time; and an OR circuit to receivethe duty control signal and a signal from the delay circuit.
 6. TheDC-DC converter according to claim 1, wherein the pulse width controlcircuit further comprise: a first circuit to reduce the pulse width ofthe duty control signal output from the PWM comparison circuit by apredetermined amount; a second circuit to increase the pulse width ofthe duty control signal output from the PWM comparison circuit by apredetermined amount; and a selector to select and output one of signalsoutput from the first circuit and the second circuit, the selector beingcontrolled based on the output voltage and an input voltage.